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An airliner has made an emergency landing in the United States after a gaping hole in the roof caused a sudden drop in cabin pressure.

Southwest Airlines flight 812 from Phoenix to Sacramento put down safely at a military air base in Arizona.

Passengers heard a bang as the roof panel blew open in the cabin. "You can see completely outside," one woman told the Associated Press news agency.

It was not immediately clear what caused the fuselage to rupture.

An FBI spokesman said terrorism was not suspected and "it appears to be a mechanical issue".

Passenger Brenda Reese said the Boeing 737 plane had just left Phoenix when she heard a "gunshot-like sound".

"It's at the top of the plane, right up above where you store your luggage," she told AP by telephone from the plane.

"The panel's not completely off. It's like ripped down... When you look up through the panel, you can see the sky."

She said the plane "dropped pretty quick".

Another woman on board said: "They had just taken drink orders when I heard a huge sound and oxygen masks came down.

"There was a hole in the fuselage about three feet long. You could see the insulation and the wiring. You could see a tear the length of one of the ceiling panels," the woman told a local television station.

Aviation officials said the pilot made a rapid, controlled descent.

The National Transportation Safety Board said an "in-flight fuselage rupture" led to the sudden descent and drop in cabin pressure.

"We do not know the cause of the decompression," Federal Aviation Administration spokesman Ian Gregor said.

Southwest Airlines said no injuries were reported among the 118 passengers on board. One flight attendant was slightly injured, the statement said.